The invention relates to a method for deforming a hollow workpiece having at least one open end, such as a metal cylinder, for example, wherein the workpiece is clamped down in a clamping device, a first forming tool is placed into contact with the outer surface of the workpiece, the workpiece and the tool are rotated about an axis of rotation relative to each other and the workpiece is deformed by means of said first tool. A method and forming machine of this kind are known for example from European patent application No. EP 0 916 428. Said publication discloses a method and a forming machine, comprising a forming head fitted with a number of rollers, by means of which the diameter of one end of a cylindrical metal element is reduced and moreover bent through an angle.
To this end, the metal cylinder is clamped down and said cylinder and said forming head are rotated relative to each other about an axis of rotation, whereupon said end is deformed by pressing said rollers in a radial direction against the outer surface of said cylinder and moving them along said outer surface in a number of cycles, whereby the radial distance between the rollers and the axis of rotation is decreased with each cycle, as a result of which a reduction of the diameter is obtained. Since the axis of rotation is at an angle with the central axis of the metal cylinder, the end of the cylinder is not only reduced as a result of the movement in radial direction of the rollers, but in addition said end will also be positioned at an angle. Due to the use of the aforesaid cycles, the workpiece assumes the shape of the final product step by step.
EP 0 916 426 discloses a comparable method and forming machine, wherein the axis of rotation is eccentrically offset from the central axis of the metal cylinder. Thus a product is obtained wherein the central axis of the deformed portion is likewise offset from the central axis of the undeformed portion of the metal cylinder.
The method and apparatus in hand can be used, for example, in the production of the housings of catalytic converters that form part of the exhaust system of vehicles, such as passenger cars. Such catalytic converters have a diameter which is larger than the diameter of the pipes of the exhaust system of which they form part, and they are preferably positioned close to the engine block in order to reach their operating temperature as quickly as possible after the engine has been started and to maintain that temperature as much as possible. One consequence of this is that, first of all, the diameter of the connections on either side of the catalytic converter housing must be reduced in order to properly connect to the rest of the exhaust system and that in addition they often need to have a complicated shape in order to enable an optimum position with respect to the engine block.
Prior art methods and apparatuses for producing workpieces having at least one deformed end, such as e.g. the above-described catalytic converter housings, appear to provide insufficient freedom as regards to shaping. Moreover, it is necessary to use relatively thick-walled workpieces, since a heavy load is exerted on the material during the deformation process, which may lead to folds, cracks and/or an irregular distribution of the wall thickness of the final product. As a result, the obtained products, such as e.g. the aforesaid catalytic converter housings, are often heavier than is necessary for their proper functioning in an exhaust system.